lib: parser: use reentrant mode on flex & bison

lib: parser: use reentrant mode on flex & bison

This removes remaining global variables from the lexer, pushing the
lexer state into struct parser_ctx too.  At the same time, "cmd_yy" is
used as prefix for all parser & lexer routines.

The result is that (a) there is no collision anymore if a program uses
flex/bison and links libzebra, and (b) the parser is fully encapsulated
and could be called in parallel from multiple threads.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>