completely rewrote file_key_management plugin

numerous issues fixed:
* buffer overflows
* error conditions aren't checked (crash if file doesn't exist)
* accessing random unallocated memory
* hard-coded password
* arbitrary hard-coded key id limit
* incomprehensible error messages (for key_id == 0 it reported
  "The key could not be initialized", for syntax errors the message was
  "Wrong match of the keyID, see the template", for a key id
  larger than hard-coded limit the message was "No asked key", and there
  was an error "Is comment" for a comment).
* tons of small mallocs, many are freed few lines down in the code
* malloc(N) and new char[N] are used both, even in the same function
* redundant memory copies
* pcre - "I can solve it with regular expressions" - with incorrect regexes
* parser context stored in a singleton
* keys are stored as strings and are strlen-ed and hex2bin-ed on every
  get_key() request
* lots of useless code (e.g. sprintf instead of a pointer assignment,
  checking of the file length to read a part of it in a fixed buffer,
  multiplying by sizeof(char) in many places, etc)
* this list is not exhaustive

mysql-test/std_data/keys.txt

@@ -1,6 +1,6 @@
-1;F5502320F8429037B8DAEF761B189D12;770A8A65DA156D24EE2A093277530142
-2;35B2FF0795FB84BBD666DB8430CA214E;4D92199549E0F2EF009B4160F3582E5528A11A45017F3EF8
-3;7E892875A52C59A3B588306B13C31FBD;B374A26A71490437AA024E4FADD5B497FDFF1A8EA6FF12F6FB65AF2720B59CCF
-4;021B0663D4DD7B54E2EBC852677E40BD;18420B5CBA31CCDFFE9716E91EB61374D05914F3ADE23E03
-5;9BF92CEA026CE732DA80821122A8CE97;966050D7777350B6FD5CCB3E5F648DA45C63BEFB6DEDDFA13443F156B7D35C84
-6;BC44D4AFD2D9FCD82A679E4DC6700D06;B5EA210C8C09EF20DB95EC584714A89F
+1;770A8A65DA156D24EE2A093277530142
+2;4D92199549E0F2EF009B4160F3582E5528A11A45017F3EF8
+3;B374A26A71490437AA024E4FADD5B497FDFF1A8EA6FF12F6FB65AF2720B59CCF
+4;18420B5CBA31CCDFFE9716E91EB61374D05914F3ADE23E03
+5;966050D7777350B6FD5CCB3E5F648DA45C63BEFB6DEDDFA13443F156B7D35C84
+6;B5EA210C8C09EF20DB95EC584714A89F