cmake: add linter, fix issues

Ref: https://cmake-format.readthedocs.io/en/latest/cmake-lint.html
Ref: https://github.com/cheshirekow/cmake_format

Closes #1610

ci/cmakelint.sh

@@ -0,0 +1,55 @@
+#!/bin/sh
+# Copyright (C) Dan Fandrich, <dan@coneharvesters.com>, Viktor Szakats, et al.
+#
+# SPDX-License-Identifier: curl
+
+# https://cmake-format.readthedocs.io/en/latest/cmake-lint.html
+# https://cmake-format.readthedocs.io/en/latest/lint-usage.html
+# https://github.com/cheshirekow/cmake_format/blob/master/cmakelang/configuration.py
+
+# Run cmakelint on the curl source code. It will check all files given on the
+# command-line, or else all relevant files in git, or if not in a git
+# repository, all files starting in the tree rooted in the current directory.
+#
+# cmake-lint can be installed from PyPi with the command "python3 -m pip
+# install cmakelang".
+#
+# The xargs invocation is portable, but does not preserve spaces in file names.
+# If such a file is ever added, then this can be portably fixed by switching to
+# "xargs -I{}" and appending {} to the end of the xargs arguments (which will
+# call cmakelint once per file) or by using the GNU extension "xargs -d'\n'".
+{
+  if [ -n "$1" ]; then
+    for A in "$@"; do printf "%s\n" "$A"; done
+  elif git rev-parse --is-inside-work-tree >/dev/null 2>&1; then
+    git ls-files
+  else
+    # strip off the leading ./ to make the grep regexes work properly
+    find . -type f | sed 's@^\./@@'
+  fi
+} | grep -E '(^CMake|/CMake|\.cmake$)' | grep -v -E '(\.h\.cmake|\.in|\.c)$' \
+  | xargs \
+  cmake-lint \
+    --suppress-decorations \
+    --disable \
+    --line-width 132 \
+    --tab-size 2 \
+    --use-tabchars false \
+    --disabled-codes C0113 \
+    --function-pattern 'libssh2_[0-9a-z_]+' \
+    --macro-pattern 'libssh2_[0-9a-z_]+' \
+    --global-var-pattern '[A-Z][0-9A-Z_]+' \
+    --internal-var-pattern '_[a-z][0-9a-z_]+' \
+    --local-var-pattern '_[a-z][0-9a-z_]+' \
+    --private-var-pattern '_[0-9a-z_]+' \
+    --public-var-pattern '([A-Z][0-9A-Z_]+|[A-Z][A-Za-z0-9]+_FOUND|[a-z]+_SOURCES|prefix|exec_prefix|includedir|libdir)' \
+    --argument-var-pattern '_[a-z][0-9a-z_]+' \
+    --keyword-pattern '[A-Z][0-9A-Z_]+' \
+    --max-conditionals-custom-parser 2 \
+    --min-statement-spacing 1 \
+    --max-statement-spacing 2 \
+    --max-returns 6 \
+    --max-branches 12 \
+    --max-arguments 5 \
+    --max-localvars 15 \
+    --max-statements 50