Remove feature HAVE_QSORT

qsort conforms to C99, so we don't need our own version.
remove HAVE_SETJMP_H
remove HAVE_ERRNO
remove HAVE_BCMP
remove HAVE_MEMCMP
remove HAVE_MEMSET
remove HAVE_MATH_H
remove HAVE_INTTYPES
remove HAVE_STDINT
remove HAVE_STDARG_H
remove HAVE_{ISWUPPER,ISWLOWER,TOWUPPER,TOWLOWER}
enable coveralls support
Move simplify_filename from tag.c
Move functions from window.c
Move fix_fname from buffer.c
Move after_pathsep from misc2.c
Move same_directory from misc2.c
Move pathcmp from misc2.c
Move mch_expandpath from os_unix.c
Move shorten_fname{,1,s} and shorten_filenames from fileio.c