install: fix absolute symbolic permissions

[mknos]

• When using symbolic permissions with '=', the resulting file permissions weren't right; here's what I found...
• mod() should be passed the original file to stat(), but it was being passed the new target file after File::Copy::copy() created it
• Base permissions for "absolute" symbolic permissions (i.e. those set with '=') are zero, so it is not correct to stat() the original file in this case
• The statement clearing bits in %perms for operator '=' in loop should only clear bits for $who the current iteration looks at

%perl install -m u=rwx,g=rx,o=x comm a/comm # -m 751
%perl install -m u=rwx,g=r comm a/comm # -m 740
%perl install -m 'a=' comm a/comm # -m 0

[coveralls]

Pull Request Test Coverage Report for Build 12041873260

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 73.805%

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Totals
Change from base Build 12040230006:	0.7%
Covered Lines:	355
Relevant Lines:	481

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💛 - Coveralls