Import bmake-20230126

Relevant/interesting changes (see ChangeLog for more):

o variables like .newline and .MAKE.{GID,PID,PPID,UID}
should be read-only.
o .[NO]READONLY: for control of read-only variables
o .SYSPATH: for controlling the path searched for makefiles
o allow for white-space between command specifiers @+-
o add more details to warning 'Extra targets ignored'
o make.1: sync list of built-in variables with reality
sort list of built-in variables
o cond.c: add more details to error message for numeric comparison
o job.c: fix handling of null bytes in output
o Allow .break to terminate a .for loop early
o var.c: fix out-of-bounds errors when parsing
o fix exit status for '-q' (since 1994)