Move psql's psqlscan.l into src/fe_utils.

This completes (at least for now) the project of getting rid of ad-hoc
linkages among the src/bin/ subdirectories.  Everything they share is now
in src/fe_utils/ and is included from a static library at link time.

A side benefit is that we can restore the FLEX_NO_BACKUP check for
psqlscanslash.l.  We might need to think of another way to do that check
if we ever need to build two lexers with that property in the same source
directory, but there's no foreseeable reason to need that.

src/include/fe_utils/psqlscan.h

@@ -0,0 +1,85 @@
+/*-------------------------------------------------------------------------
+ *
+ * psqlscan.h
+ *	  lexical scanner for SQL commands
+ *
+ * This lexer used to be part of psql, and that heritage is reflected in
+ * the file name as well as function and typedef names, though it can now
+ * be used by other frontend programs as well.  It's also possible to extend
+ * this lexer with a compatible add-on lexer to handle program-specific
+ * backslash commands.
+ *
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/fe_utils/psqlscan.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PSQLSCAN_H
+#define PSQLSCAN_H
+
+#include "pqexpbuffer.h"
+
+
+/* Abstract type for lexer's internal state */
+typedef struct PsqlScanStateData *PsqlScanState;
+
+/* Termination states for psql_scan() */
+typedef enum
+{
+	PSCAN_SEMICOLON,			/* found command-ending semicolon */
+	PSCAN_BACKSLASH,			/* found backslash command */
+	PSCAN_INCOMPLETE,			/* end of line, SQL statement incomplete */
+	PSCAN_EOL					/* end of line, SQL possibly complete */
+} PsqlScanResult;
+
+/* Prompt type returned by psql_scan() */
+typedef enum _promptStatus
+{
+	PROMPT_READY,
+	PROMPT_CONTINUE,
+	PROMPT_COMMENT,
+	PROMPT_SINGLEQUOTE,
+	PROMPT_DOUBLEQUOTE,
+	PROMPT_DOLLARQUOTE,
+	PROMPT_PAREN,
+	PROMPT_COPY
+} promptStatus_t;
+
+/* Callback functions to be used by the lexer */
+typedef struct PsqlScanCallbacks
+{
+	/* Fetch value of a variable, as a pfree'able string; NULL if unknown */
+	/* This pointer can be NULL if no variable substitution is wanted */
+	char	   *(*get_variable) (const char *varname, bool escape, bool as_ident);
+	/* Print an error message someplace appropriate */
+	/* (very old gcc versions don't support attributes on function pointers) */
+#if defined(__GNUC__) && __GNUC__ < 4
+	void		(*write_error) (const char *fmt,...);
+#else
+	void		(*write_error) (const char *fmt,...) pg_attribute_printf(1, 2);
+#endif
+} PsqlScanCallbacks;
+
+
+extern PsqlScanState psql_scan_create(const PsqlScanCallbacks *callbacks);
+extern void psql_scan_destroy(PsqlScanState state);
+
+extern void psql_scan_setup(PsqlScanState state,
+				const char *line, int line_len,
+				int encoding, bool std_strings);
+extern void psql_scan_finish(PsqlScanState state);
+
+extern PsqlScanResult psql_scan(PsqlScanState state,
+		  PQExpBuffer query_buf,
+		  promptStatus_t *prompt);
+
+extern void psql_scan_reset(PsqlScanState state);
+
+extern void psql_scan_reselect_sql_lexer(PsqlScanState state);
+
+extern bool psql_scan_in_quote(PsqlScanState state);
+
+#endif   /* PSQLSCAN_H */