Client and core inappropriately use reserved exit statuses

[arisu3]

The client and core misuse exit status 126 to indicate an unavailable GPU and status 127 to indicate a stalled WU:

fah-client-bastet/src/fah/client/ExitCode.h

Lines 164 to 168 in ca69e57

	/// v710, FAIL:
	CBANG_ENUM_VALUE(GPU_UNAVAILABLE_ERROR, 126)
	/// v745, FAIL:
	CBANG_ENUM_VALUE(WU_STALLED, 127)

It also misuses status 255 to indicate a miscellaneous failure:

fah-client-bastet/src/fah/client/ExitCode.h

Lines 170 to 171 in ca69e57

	/// v623, FAIL:
	CBANG_ENUM_VALUE(FAILED_3, 255)

This violates POSIX.1-2024 § 2.8.2 which reserves status 126 for a command that is found but not executable, status 127 for a command that could not be found (or which requires libraries that could not be found), and status 128 and above for fatal signals. Note that ld-linux.so will use status 126 and 127 even if the command is executed directly and a POSIX shell is not involved.

As a result of this misuse, errors involving missing libraries or permission errors cause the client to dump the WU with a misleading message that is confusing to users. As many of the cores have issues with missing libraries, this results in a large number of dumped WUs (tens of thousands of dumped WUs in #266, whose problem that was misidentified in #266 (comment) as a transient error that "just happened" to return status 127).

The only exit statuses that are permitted for application use are 0-125, inclusive.

I have not made a client PR because a PR for the core is also needed, and I have no access to the core's code. The change should be trivial, though, and could be easily made in time for core 0x28 with a patch of only a few lines.

[arisu3]

Pending fixes to the closed source cores, this could be fixed (saving tens of thousands of unnecessarily dumped WUs) on the client by having it run the core with a dummy argument such as --help and ignoring the output, then checking to see if the exit status is 127 and, if so, print a diagnostic message and pause any attempts at downloading a new WU.