Extract strings.c from misc2.c
Move `concat_strings` from path.c
Remove two-iteration loop from ExpandGeneric
Remove two-iteration loop from expand_filename
Remove two-iteration loop from check_keyword_id
Remove checks for SHORT_FNAME and USE_LONG_FNAME.

They were only defined for MSDOS which is now unsupported.
Don't retry to make the tempname in write_viminfo() with shortname=1

ref #572
Remove the code related to the shortname option
Remove the always-FALSE shortname argument from buf_modname()
Rename buf_modname to modname as they've become the same
Update README: progress section
Improve test_eval.in

The old version required to change the range given to :w for each new
test. Now a new buffer is used instead.
replace some mch_stat() with os_file_exists()
use os_getperm in f_getfperm
rename buf_store_time to buf_store_file_info
implement os_file_get_size
implement FileInfo struct
implemented os_get_file_info_fd
implemented os_get_file_info_link()
replaced mch_fstat() with os_file_get_info_fd()