gzip: drop mentions of Amiga, VMS

gzip: drop mentions of Amiga, VMS

These platforms were not supported anyway, and their code was
suffering from bitrot.  This patch may also fix some file name
glitches on MS-Windowsish platforms.
* bootstrap.conf (gnulib_modules): Add dosname.
* gzip.c: Include dosname.h.
(shorten_name, treat_dir): Use last_component rather than looking
at path separators by hand.
* tailor.h: Remove sections on porting to VMS and to Amiga,
since dosname.h doesn't support these platforms anyway.
(PATH_SEP2, PATH_SEP3, STDC_HEADERS, SUFFIX_SEP, RECORD_IO)
(HAVE_CHOWN, HAVE_LSTAT, HAVE_SYS_DIR_H, direct): Remove.  All
uses removed.  Many uses replaced by calls to ISSLASH and/or
last_component.