Add mach_error_string (and mach_error_t)

[stepancheg]

mach_error_string is defined in /usr/include/mach/mach_error.h
It is not referenced in the documentation Apple website.

char            *mach_error_string(
/*
 *  Returns a string appropriate to the error argument given
 */
    mach_error_t error_value
    );

mach_error_t is defined in /usr/include/mach/error.h
https://developer.apple.com/documentation/kernel/mach_error_t

typedef kern_return_t   mach_error_t;

[rustbot]

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[tgross35]

Thanks for the PR, could you update the description with links to the headers and/or docs?

[stepancheg]

Done.

[tgross35]

Thanks! For reference the opensource.apple.com headers or xnu are good places to link to for apple stuff, e.g. this one is here https://opensource.apple.com/source/Libc/Libc-166/mach.subproj/mach_error_string.c.auto.html and here https://github.com/apple-oss-distributions/xnu/blob/8d741a5de7ff4191bf97d57b9f54c2f6d4a15585/libsyscall/mach/mach_error_string.c#L177-L183.