add ability to specify heap size hint as a percent

[JeffBezanson]

also fix overflowed subtraction with size hints < 250mb --- we should pick some minimum soft heap limit; I chose 1mb here but it could be anything.

[vtjnash]

LGTM

[KristofferC]

Bump, can be merged?

[vtjnash]

No, it failed on CI:

2024-02-01 04:27:38 UTC	Error in testset cmdlineargs:
2024-02-01 04:27:38 UTC	Test Failed at C:\buildkite-agent\builds\win2k22-amdci6-2\julialang\julia-master\julia-ec4182e384\share\julia\test\cmdlineargs.jl:1059
2024-02-01 04:27:38 UTC	  Expression: abs(maxmem - (hint - 250 * 1024 * 1024)) / maxmem < 0.01
2024-02-01 04:27:38 UTC	   Evaluated: 4.170485178629558 < 0.01

[JeffBezanson]

Seems I did not account for 32-bit machines with 32gb of memory 🤔 😕

[vtjnash]

Nobody ever does, until they realize it is probably the most common 32 bit machines now, haha