[2.6] Backport parser fixes from 3.3.0 - #325

Note that this DOES NOT implement identical parsing to in 3.x. Every attempt has been made to ensure this backport behaves exactly as in 2.5.x, and also allows cases that previously crashed in 2.5.x, being as lenient as possible (where identical case in 3.3.x will hard fail).

This fixes the example from #332.

How does this differ from the parsing in 3.3+:

1. Unquoted values without spaces starting with a hash are considered to be comments in 3.3+ but in 2.x are treated as literal values.
2. Invalid escape sequences within quoted values in 2.5 fail silently, in 2.6 are permitted, and in 3.x will fail loudly (with a proper error message in 3.3+).

Example 1:

FOO=#foo

2.5: parses as #foo
2.6: parses as #foo
3.2: parses as #foo (considered a bug in the 3.x series)
3.3: parses as the empty string

Example 2:

FOO=#foo bar

2.5: parses as the empty string
2.6: parses as the empty string
3.2: parses as the empty string
3.3: parses as the empty string

Example 3:

FOO="#foo"

2.5: parses as #foo
2.6: parses as #foo
3.2: parses as #foo
3.3: parses as #foo

Example 4:

FOO="iiiiviiiixiiiiviiii\n"

2.5: fails silently
2.6: parses as iiiiviiiixiiiiviiii\n
3.2: fails with an obscure preg error
3.3: fails saying invalid escape sequence

Example 5:

FOO="iiiiviiiixiiiiviiii\\n"

2.5: parses as iiiiviiiixiiiiviiii\n
2.6: parses as iiiiviiiixiiiiviiii\n
3.2: parses as iiiiviiiixiiiiviiii\n
3.3: parses as iiiiviiiixiiiiviiii\n