options: fix buffer overrun

Google's OSS-fuzz turned up a buffer overrun with value of the filename
option due to an overrun in a MAX_PATH sized buffer. To reproduce
compile fio with address sanitizer options like the following

LDFLAGS="-fsanitize=address" ./configure --disable-optimizations \
      --extra-cflags="-fsanitize=address"

The issue is demonstrated by the following job:

% COUNT=$(getconf PATH_MAX /); printf "[t]\nfilename=%${COUNT}s" \
  | sed 's/ /@/g' | fio --parse-only -
=================================================================
==45748==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffee8e35780 at pc 0x00010735a343 bp 0x7ffee8e35270 sp 0x7ffee8e34a08
WRITE of size 1025 at 0x7ffee8e35780 thread T0
    #0 0x10735a342 in wrap_vsprintf (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x22342)
    #1 0x10735a9ac in wrap_sprintf (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x229ac)
    #2 0x106e83b01 in add_file filesetup.c:1656
    #3 0x106ee8c87 in str_filename_cb options.c:1320
    #4 0x106ee1b44 in __handle_option parse.c:792
    #5 0x106ed99ad in handle_option parse.c:1014
    #6 0x106eda07d in parse_option parse.c:1184
    #7 0x106ef10ea in fio_options_parse options.c:5199
    #8 0x106e27684 in __parse_jobs_ini init.c:2076
    #9 0x106e25377 in parse_jobs_ini init.c:2127
    #10 0x106e2c971 in parse_options init.c:2989
    #11 0x106ffc884 in main fio.c:42
    #12 0x7fff702f1cc8 in start (libdyld.dylib:x86_64+0x1acc8)

Address 0x7ffee8e35780 is located in stack of thread T0 at offset 1056 in frame
    #0 0x106e836ef in add_file filesetup.c:1644

  This frame has 1 object(s):
    [32, 1056) 'file_name' (line 1646) <== Memory access at offset 1056 overflows this variable

Return an error message to the user by doing the following:

- Allow "regular" string options to have a maxlen parameter
- Set the filename option to have a maxlen of MAX_PATH

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>