Gawk built-in functions shadowing

[xonixx]

Relates to #69

Understand what to do:

$ ./soft/gawk51 'function foo(gsub) { print gsub } BEGIN { foo(123) }'
gawk51: cmd. line:1: function foo(gsub) { print gsub } BEGIN { foo(123) }
gawk51: cmd. line:1:              ^ syntax error
gawk51: cmd. line:1: function foo(gsub) { print gsub } BEGIN { foo(123) }
gawk51: cmd. line:1:                                 ^ syntax error

 $ ./soft/gawk51 'function foo(gensub) { print gensub } BEGIN { foo(123) }'
123

$ awk 'function foo(gensub) { print gensub } BEGIN { foo(123) }'
awk: cmd. line:1: function foo(gensub) { print gensub } BEGIN { foo(123) }
awk: cmd. line:1:              ^ syntax error
awk: cmd. line:1: function foo(gensub) { print gensub } BEGIN { foo(123) }
awk: cmd. line:1:                                     ^ syntax error

$ awk --version
GNU Awk 4.1.3, API: 1.1 (GNU MPFR 3.1.4, GNU MP 6.1.0)

$ ./soft/gawk51 --posix 'BEGIN { and = 123; print and }'
123

 $ ./soft/gawk51 'BEGIN { and = 123; print and }'
gawk51: cmd. line:1: BEGIN { and = 123; print and }
gawk51: cmd. line:1:             ^ syntax error
gawk51: cmd. line:1: BEGIN { and = 123; print and }
gawk51: cmd. line:1:                              ^ syntax error

$ ./soft/bwk 'BEGIN { and = 123; print and }'
123

$ ./soft/gawk51 'BEGIN { gensub = 123; print gensub }'
gawk51: cmd. line:1: BEGIN { gensub = 123; print gensub }
gawk51: cmd. line:1:                ^ syntax error
gawk51: cmd. line:1: BEGIN { gensub = 123; print gensub }
gawk51: cmd. line:1:                                    ^ syntax error

$ ./soft/gawk51 '@namespace "a" ; BEGIN { gensub = 123; print gensub }'
123

$ ./soft/gawk51 '@namespace "a" ; BEGIN { gsub = 123; print gsub }'
gawk51: cmd. line:1: @namespace "a" ; BEGIN { gsub = 123; print gsub }
gawk51: cmd. line:1:                               ^ syntax error
gawk51: cmd. line:1: @namespace "a" ; BEGIN { gsub = 123; print gsub }
gawk51: cmd. line:1:                                                 ^ syntax error

Parsing rules change depending on namespace:

$ ./soft/gawk51 'BEGIN { print mktime ("2021 01 01 01 01 01") }'
1609455661

$ ./soft/gawk51 '@namespace "a"; BEGIN { print mktime ("2021 01 01 01 01 01") }'
2021 01 01 01 01 01

$ ./soft/gawk51 '@namespace "awk"; BEGIN { print mktime ("2021 01 01 01 01 01") }'
1609455661

It's clear why this is done for Gawk - to not break the old scripts intended to work on Posix Awk. But how can we handle this better? In the naive approach we'll just fail to parse function f(and) {} because and is a Gawk bitwise function.

Relevant doc https://www.gnu.org/software/gawk/manual/html_node/Naming-Rules.html