[SV 63667] In .POSIX, use shell flags -c when ignoring errors

[SV 63667] In .POSIX, use shell flags -c when ignoring errors

* src/variable.c (lookup_variable_for_file): New function to retrieve
a variable assignment in a file context.
* src/variable.h (lookup_variable_for_file): Declare it.
* src/job.c (construct_command_argv): Look up .SHELLFLAGS.  If .POSIX
is set and we're using the default value, choose -c if we're ignoring
errors else choose -ec.
(construct_command_argv_internal): Ditto.
* tests/scripts/targets/POSIX: Add tests.