Fewer uses of uintmax_t in xheader.c

Fewer uses of uintmax_t in xheader.c

* src/tar.h (struct xheader):
* src/xheader.c (xheader_string_end):
Use idx_t, not uintmax_t, for string length.
* src/xheader.c (xheader_string_add):
Avoid duplicate calls to strlen.
(xheader_string_end): Remove by-hand check for size overflow;
it’s not possible, as this is measuring allocated storage.