Begin work on MySQL CNID backend

o add configure check --with-mysql-config=PATH that searches for
  the mysql-config binary
o add new MySQL CNID subdirectory and compilation infrastructure
o modify CNID header files for new backend
o add private CNID MySQL header file for private data
o add mysql config options
o add MySQL args to cnid_open()
o add support for MySQL CNID backend to dbd

Add MySQL libraries as deps to executables:
workaround libtool bug where the rpath of a dependent library (ie mysql) of a libtool
library (ie libatalk) gets stripped from the link flags when linking the final
executable, even though the library (mysql) is a non default OS dir.
Add vasprintf for OSes missing it
Modify lookup to use a single OR query
Enhance lookup to use prepared statements
Enhance cnid_add with prepared statements
Include SQL query in error message
Add MYSQL_CFLAGS and MYSQL_LIBS with AC_ARG_VAR
Modify update behaviour to delete everythig and then insert in a loop until it worked
Fix mysql_stmt_fetch() result check and reset the hint in the db handle
Move MySQL check to file with the other CNID backend checks and add options summary for afpd