[Fix] nvm use: do not clobber man's default search path

`man` consults its own configured search path only when MANPATH holds an empty entry;
a list without one replaces the default outright.
`nvm use` produced exactly that,
so activating a version could hide every system man page.

Contribute the empty entry as a trailing one,
so that nvm's directory keeps precedence over the system's,
and only when the list has none already,
so that repeated `nvm use` calls are idempotent and an empty entry the user placed stays where they put it.

The `$(manpath)` snapshot this replaces went stale the moment man's configuration changed,
and cost a subprocess per `nvm use`.
It was also dead: `local MANPATH` scoped the assignment to `nvm()`,
so `export` did not outlive the call and MANPATH went untouched whenever it started out unset.
Only ksh, where `local` is not a builtin, ever ran it.

`nvm deactivate` now unsets MANPATH where nvm's was the only real entry,
rather than leaving a bare `:` behind.

Reported and diagnosed by @al0ksar in #3890, which used a leading empty entry instead.
That one accrues one more colon on every `nvm use`,
which `nvm deactivate` then leaves behind,
and it loses to the system's man pages wherever nvm's bin directory is not first on PATH.

Supersedes #2077.

Refs #3890
Refs #2077

README.md

@@ -870,7 +870,7 @@ nvm exposes the following environment variables:
 - `NVM_CD_FLAGS` - used to maintain compatibility with zsh.
 - `NVM_RC_VERSION` - version from .nvmrc file if being used.
 
-Additionally, nvm modifies `PATH`, and, if present, `MANPATH` and `NODE_PATH` when changing versions.
+Additionally, nvm modifies `PATH` when changing versions, along with `MANPATH` wherever a `manpath` command exists, and `NODE_PATH`, if present.
 
 The following environment variables can be set to configure `nvm install`: