Remove some uses of global variable gram_pos.

Remove some uses of global variable gram_pos.

* gettext-tools/src/msgfmt.c (msgfmt_comment_special): Use the file name from
the catalog reader instead of gram_pos.
* gettext-tools/src/read-desktop.c (gram_pos): Remove declaration.
(pos): New variable.
(phase2_getc, phase2_ungetc, desktop_lex, desktop_parse): Use it instead of
gram_pos.
* gettext-tools/src/read-properties.c (gram_pos): Remove declaration.
(pos): New variable.
(phase2_getc, phase2_ungetc, phase4_getuc, read_escaped_string,
properties_parse): Use it instead of gram_pos.
* gettext-tools/src/read-stringtable.c (gram_pos): Remove declaration.
(pos): New variable.
(phase3_getc, phase3_ungetc, stringtable_parse): Use it instead of gram_pos.
(read_string): Likewise. Rename a parameter.
(conv_from_ucs4): Rename a local variable.