ini: Fix parsing of variable with spaces before the =

At least from what the source code suggests, a variable declaration in
the config file as this should be supported:
DFLAGS = -I/usr/include/d

But in reality, if you live spaces like that, the variable value will
get uppercased.

This commit fixes this problem to properly parse the extra spaces.
man: Mention variable names are converted to uppercase
ini: Add support to define undefined variables

Similarly to Make files, the syntax `VAR ?= value` is added to define
variables only when they were previously undefined.

In this case, if the environment variable `VAR` was undefined, then it
will be defined with the value `value`, otherwise the previous content
of `VAR` will be preserved.

This is useful to define default flags in the config file that can be
easily overridden by using environment variables.
fix Issue 12047 - UDAs are not checked

This is also a refactoring change for UDA semantic analysis.

1. Run UDAs in semantic2 phase.
Currently forward reference resolution mechanism is not perfect. UDAs often depends on other declarations in user code. Therefore analysing them in semantic1 phase would easily hit fwdref issues.
On the other hand, if UDAs are not checked for conditional compilation, essentially they have no effect for final execution code. So, I think moving their analysis timing from semantic1 to semantic2 is not a problem.

2. Each Dsymbols now have a pointer to the enclosing UserAttributeDeclaration.
We can use UserAttributeDeclaration::scope for forward reference resolution of UDAs. So we can elide Dsymbol::userAttributesScope.
Move some comments in front of the declaration they describe
Put comment before if instead of inside condition
Merge pull request #3197 from yebblies/doccomment

[DDMD] [trivial] Put comment before if instead of inside condition
Remove comments inside array initializers
Merge pull request #3199 from yebblies/listcomment

[DDMD] Remove comments inside array initializers
Merge pull request #3198 from yebblies/infront

[DDMD] Move some comments in front of the declaration they describe
Put braces around comments preceeding if and else bodies
Remove block of dead code
Remove some comments inside expressions
Merge pull request #3202 from yebblies/deadblock

[DDMD] Remove block of dead code
Merge pull request #3201 from yebblies/expcomment

[DDMD] Remove some comments inside expressions
Merge pull request #3200 from yebblies/bracecomment

[DDMD] Put braces around comments preceeding if and else bodies
The frontend should not know or care which array ops are implemented in druntime
Merge pull request #3203 from yebblies/arrayglue

The frontend should not know or care which array ops are implemented in druntime
Move windows С++ mangle to frontend
fix Issue 11824 - A stack variable escaping problem in CTFE Phobos code