Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

Replace all known uses of ln in the build process with appropriate
install -l invocations via new INSTALL_LINK and INSTALL_SYMLINK
variables.

Sponsored by: DARPA, AFRL
Reviewed by: ian, ray, rpaulo

Try really hard to fix parallel installs. Add a bunch of .ORDER
directives to ensure that all realinstall sub-tasks are executed
after beforeinstall, similarly ensure that all afterinstall sub-
tasks are executed after realinstall. Demonstration:

all: task1 task2
.ORDER: task1 task2

task2: task2_subtask
.ORDER: task1 task2_subtask

task1 task2 task2_subtask:
@sleep `jot -r 1 0 1.0`
@echo ${.TARGET}

Without the second .ORDER directive, task2_subtask can be run in
parallel with task1.

Spotted by: Andrea Campi <andrea@webcom.it>

Handle installation of hard and symbolic links via a seperate .mk file.