* Fallbacks for absent NAME_MAX and PATH_MAX

Certainly NAME_MAX isn't guaranteed to exist and on (some versions of?)
Illumos and SmartOS it doesn't, so provide some reasonably accommodating
fallbacks.

* Smaller limits

* Remove now redundant PATH_MAX definition for Hurd

Kind contribution from uno20001:

https://old.reddit.com/r/C_Programming/comments/etavho/file_manager_nnn_needs_help_with_pcre/fffk4wr/

* Experimental PCRE library support

To compile with PCRE install libpcre-dev(el) and run:

cc  -Wall -Wextra -O3 -DPCRE -D_GNU_SOURCE -D_DEFAULT_SOURCE -I/usr/include/ncursesw -I/usr/include -o nnn src/nnn.c -lreadline -lncursesw -ltinfo -lpcre

* Detach process with F_NOWAIT

Co-authored-by: Mischievous Meerkat <engineerarun@gmail.com>

To compile with PCRE install libpcre-dev(el) and run:

cc  -Wall -Wextra -O3 -DPCRE -D_GNU_SOURCE -D_DEFAULT_SOURCE -I/usr/include/ncursesw -I/usr/include -o nnn src/nnn.c -lreadline -lncursesw -ltinfo -lpcre

Both in light and detail modes we show more characters than in status bar.

This reverts commit 681b22d5c7.

The statvfs man page:
http://man7.org/linux/man-pages/man3/statvfs.3.html

says clearly:

fsblkcnt_t     f_blocks;   /* Size of fs in f_frsize units */

I missed this earlier.

Replaces the following environment variables:
- NNN_USE_EDITOR
- NNN_IDLE_TIMEOUT

This reverts commit 527e995f1f.

* Quit with an error code

* Use `Q` as "quit with err"

* Remove extra var and simplify conditions

* Revert combining conditions

* Fix formatting

* Format help with as much tabs as possible

* Catch NULL from malloc()

* Tidy up errors

* Make indentaion look pretty in git