Propagate failures caused by pool initialization errors.

This adds an option which will check if a diff is applicable without
actually applying it; equivalent to git apply --check.

apply: Fix a patch corruption related to EOFNL handling

git_apply_to_tree() cannot be used apply patches with new files. An attempt
to apply such a patch fails because git_apply_to_tree() tries to remove a
non-existing file from an old index.

The solution is to modify git_apply_to_tree() to git_index_remove() when the
patch states that the modified files is removed.

Use of apply's API can lead to an improper patch application and a corruption
of the modified file.

The issue is caused by mishandling of the end of file changes if there are
several hunks to apply. The new line character is added to a line from a wrong
hunk.

The solution is to modify apply_hunk() to add the newline character at the end
of a line from a right hunk.

The macro `apply_err` is implemented as a variadic macro, which
are not defined by C89. Convert it to a variadic function,
instead.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.