add erl option to set schedulers by percentages

For applications where measurements show enhanced performance from the use
of a non-default number of emulator scheduler threads, having to accurately
set the right number of scheduler threads across multiple hosts each with
different numbers of logical processors is difficult because the erl +S
option requires absolute numbers of scheduler threads and scheduler threads
online to be specified.

To address this issue, add a +SP option to erl, similar to the existing +S
option but allowing the number of scheduler threads and scheduler threads
online to be set as percentages of logical processors configured and
logical processors available, respectively. For example, "+SP 50:25" sets
the number of scheduler threads to 50% of the logical processors
configured, and the number of scheduler threads online to 25% of the
logical processors available. The +SP option also interacts with any
settings specified with the +S option, such that the combination of options
"+S 4:4 +SP 50:50" (in either order) results in 2 scheduler threads and 2
scheduler threads online.

Add documentation for the +SP option.

Add tests for the +SP option to scheduler_SUITE.

Add tests and documentation for two existing features of the +S option: +S
0:0 resets the scheduler thread count and scheduler threads online count to
their defaults, and specifying negative numbers for +S results in those
values being subtracted from the default values for the host.

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