lib/sh/util.sh: added PATH_rm(), which removes components from the PATH

lib/sh/util.sh: added $top_dir_abs, $top_dir_orig

bugfix: lib/sh/util.sh: command(): use `builtin command` instead of `exec` so that options like -p (reset PATH) work properly. also, the command builtin it overrides is designed to be used with more than just external commands, and command() should not impose additional limitations.

lib/sh/util.sh: added self_sys alias, which uses only system utilities (`command -p`) instead of the current PATH

lib/sh/make.sh: make(): at verbosity < 3, hide messages about making included Makefiles. this makes the make output much more readable when a Makefile contains an include statement, because there won't be a ton of log messages every time a Makefile is included. this filtering is so useful that it probably makes sense to run make for any of our Makefiles using `lib/runscripts/util.run make ...` instead of plain make. compare e.g. `make inputs/ACAD/Specimen/map.csv` (53 lines of output) and `lib/runscripts/util.run make inputs/ACAD/Specimen/map.csv` (17 lines of output, 1/3 as much).

lib/sh/util.sh: log++: before cmd: documented that you need to use "log++" instad of log++ to avoid using the log++ alias, which prepends a log_local call. omitting the quotes is generally not a problem, but when there is another command wrapping the log++, you need the "" to avoid the wrapper applying to log_local's declare call instead.

lib/sh/util.sh: log++: before cmd: documented how to use it as `log+ #` when incrementing multiple log_levels at once (this is a better method than `"log++" "log++" ...`)

lib/sh/util.sh: log++: before cmd: documented that you need to use it as `"log++" "log++"` when repeating it multiple times

lib/sh/util.sh: added filter_fd(), which encapsulates the use of >() process substitution for filtering an fd other than stdout (yes, this is possible without lots of 3>&1 1>&2 2>&3 redirections!). this can be useful e.g. to filter logging output or highlight errors.

lib/sh/util.sh: log+(): documented that with a cmd, assignments are applied just to the cmd, so log_local is not needed