ulimit512: add new compatibility utility

ulimit512: add new compatibility utility

The POSIX standard clearly specifies that the ulimit utility's `-f`
option specifies the number of blocks and that each of those "block"s
represents a 512-byte unit:

    https://pubs.opengroup.org/onlinepubs/009695399/utilities/ulimit.html

Nevertheless, some BrAin dead SHells stubbornly insist on interpreting the
argument to the `ulimit -f` option as the number of 1024-byte units instead.

This creates a severe shell script compatibility problem where the intended
limit could either be exceeded by 2x (512-byte units were passed to the
1024-byte unit version) or be limited to only half the intended value
(1024-byte units were passed to the 512-byte version) potentially causing
unwanted breakage or unintential overflow.

To provide compatibility across shells, this utility works like a ulimit
version of the env utility.  The specified command and arguments are run
optionally first setting the value of the UL_SETFSIZE value (which is ALWAYS
in 512-byte units) via an optinoal `-f <blks>` option.

A convenient `-h` option is also supported to provide full help.

This utility should not be necessary, but it is.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>