Backport grpc#20761 and grpc#20764

Bump version to v1.25.0-pre1

Bump up BoringSSL version number

After closing a file descriptor honor the "closed" flag to avoid
re-closing it in post-fork processing.

File descriptors must be closed during an orphan operation, because
the closing of the file-descriptor is necessary for the correct function
of code that is polling on the descriptor.  Thus even if there are active
references, the close() call is necessary.  But this means that post-fork
code may close the file-descriptor, since it is only unregistered from
post-fork after the ref-count reaches 0.  All of this can be handled by
ensuring that the post-fork code honors the "close" flag.

Port grpc#20767: Add Objective-C Doxygen Documentation

Backport grpc#20452 to fix the forking issue on macOS

…-dockerfiles

PHP: backport dockerfile fixes

Bump version to v1.25.0

Port grpc#20977: gRPC-C++ podspec follows gRPC versioning

Port grpc#20988

Backport grpc#21165: Fix typo in gRPC C++ podspec