tools/exec: Implement basic environment overrides.

VAL=xxx... and VAL=$VAL:xxx... are supported; all other syntaxes
will fail with an error message.

When combined with a build/jam patch that will come in a later
commit, this makes it possible to build a large number of targets
using exec as JAMSHELL; including all of libroot. The performance
difference is extremely obvious:

jam -j2 libroot, JAMSHELL=/bin/sh (32-bit Haiku)
real 1m43.571s
user 1m10.961s
sys 1m7.965s

jam -j2 libroot, JAMSHELL=exec
real 1m28.364s
user 0m58.190s
sys 0m57.563s

So that is a savings of 15.21 seconds, or 15% of the build time.
Something that is less I/O bound and more fork-bound (e.g.
linking application catalogs) will almost certainly see
an even bigger performance difference.

Changes to add the necessary JAMSHELL overrides for those
targets which need it, in order to make it possible to
enable usage of "exec" by default, will be coming
over the next few days/weeks...

tools/exec: Exit with an error upon attempting to run multiple commands.

This way, things that need a real shell will be more clear.

tools: Add an "exec" tool.

This utility takes command-strings, e.g. "gcc -c file.c -D...",
parses them into an argv, and then execvp()s that. The use-case
is Jam, which cannot do this itself, but instead simply calls
JAMSHELL (usually just "/bin/sh -c") to do that for it.

Shells in general have a large amount of overhead (and bash in
particular is especially bad here), so using a utility like this
as JAMSHELL in most cases can be a significant speed-up.

For example, on Haiku (32-bit):

$ time sh -c 'for i in {1..100}; do sh -c "./exec test"; done'
real 0m3.335s
user 0m1.603s
sys 0m1.612s

$ time sh -c 'for i in {1..100}; do ./exec test; done'
real 0m1.547s
user 0m0.597s
sys 0m0.867s

So this means for every 100 executions, using bash has about 3.3s of
overhead, and this tool cuts out over half of that. Probably for
longer command strings, the overhead is significantly greater.
But that should be clear soon enough...