MFC r314651,r318439,r318440:

r314651:

sys/modules: normalize .CURDIR-relative paths to SRCTOP

This simplifies make output/logic

Tested with: `cd sys/modules; make ALL_MODULES=` on amd64

r318439:

Normalize .PATH on SRCTOP

This will help Jenkins dedupe 9 warnings between the static build and
the module build of ipsec(4).

Missed in SRCTOP conversion in r314651.

MFC with: r314651

r318440:

Normalize SYSDIR on SRCTOP instead of .CURDIR

This is being done to simplify pathing for CFLAGS and source files.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Follow the lead of dcphy(4) and pnphy(4) and move the reminder of the PHY
drivers that only ever attach to a particular MAC driver, i.e. inphy(4),
ruephy(4) and xlphy(4), to the directory where the respective MAC driver
lives and only compile it into the kernel when the latter is also there,
also removing it from miibus.ko and moving it into the module of the
respective MAC driver.
- While at it, rename exphy.c, which comes from NetBSD where the MAC driver
it corresponds to also is named ex(4) instead of xl(4) but that in FreeBSD
actually identifies itself as xlphy(4), and its function names accordingly
for consistency.
- Additionally while at it, fix some minor style issues like whitespace
in the register headers and add multi-inclusion protection to inphyreg.h.

Hook up new USB modules.