libgettextpo: Use a multithread-safe alternative to strerror().

libgettextpo: Use a multithread-safe alternative to strerror().

* autogen.sh (GNULIB_MODULES_LIBGETTEXTPO): Add xstrerror.
* gnulib-local/lib/markup.c: Include xstrerror.h.
(unescape_string_inplace): Use xstrerror instead of strerror.
* gnulib-local/modules/markup (Depends-on): Add xstrerror.
* gettext-tools/src/read-po-lex.c: Include xstrerror.h.
(mbfile_getc, lex_getc): Use xstrerror instead of strerror.
* gettext-tools/src/read-properties.c: Include xstrerror.h.
(properties_parse): Use xstrerror instead of strerror.
* gettext-tools/src/read-stringtable.c: Include xstrerror.h.
(phase1_getc): Use xstrerror instead of strerror.
* gettext-tools/src/write-catalog.c: Include xstrerror.h.
(msgdomain_list_print): Use xstrerror instead of strerror.