libgettextpo: Remove static variables 'buf', 'bufmax'.
* gettext-tools/src/po-gram.h (struct po_parser_state): Add fields buf, bufmax.
* gettext-tools/src/po-lex.c (lex_start): Initialize them.
(lex_end): Free the buf field.
(po_gram_lex): Use these fields instead of static variables.
