Fixup git_attr_value's comment to be recognised as documentation, and
include the definitions needed for clang to parse reset.h such that
it shows up in the documentation.

This fixes #2430.

Fixed a few header @param and @return typos with the help of -Wdocumentation in Xcode.

The following warnings have not been fixed:
common.h:213 - Not sure how the documentation format is for '...'
notes.h:102 - Correct @param name but empty text
notes.h:111 - Correct @param name but empty text
pack.h:140 - @return missing text
pack.h:148 - @return missing text

Conflicts:
	src/branch.c
	tests-clar/refs/branches/create.c

Commit 0c9eacf3d2c83256736a5bb2a240e73afd13d55f introduced the function
git_attr_value and switched the GIT_ATTR_* macros to use it, but
attempting to use that function leads to a linker error (undefined
reference to `git_attr_value').  Export git_attr_value so programs can
actually call it.

Conflicts:
	src/notes.c
	src/transports/git.c
	src/transports/http.c
	src/transports/local.c
	tests-clar/odb/foreach.c

This updates all the `foreach()` type functions across the library
that take callbacks from the user to have a consistent behavior.
The rules are:

* A callback terminates the loop by returning any non-zero value
* Once the callback returns non-zero, it will not be called again
  (i.e. the loop stops all iteration regardless of state)
* If the callback returns non-zero, the parent fn returns GIT_EUSER
* Although the parent returns GIT_EUSER, no error will be set in
  the library and `giterr_last()` will return NULL if called.

This commit makes those changes across the library and adds tests
for most of the iteration APIs to make sure that they follow the
above rules.