shellenv: prepend colon to MANPATH if set

[gromgit]

The current appended colon means system man pages always shadow Homebrew's. There's also no point adding Homebrew's man dir, nor filling out an empty MANPATH, since man and friends will add the necessary dirs according to PATH.

Closes Homebrew/homebrew-core#176037.

Also fixed a syntax error in the *csh INFOPATH setting.

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[gromgit]

I've not touched fish, because (if I understand it correctly) it auto-constructs colon-delimited values from the lists contained in all variables whose names end in PATH.

Also need help fixing the PowerShell code, which I have no experience with.

[MikeMcQuaid]

Thanks @gromgit! @Homebrew/maintainers can someone help out with Powershell and review Fish here?

[SMillerDev]

Looks good, and in my understanding this isn't applicable to fish.

[gromgit]

@jpawlowski, as the original author of the PowerShell shellenv code, can you advise on the necessary changes (do nothing if MANPATH not set, add colon at start if it is set)? Thanks much!

[MikeMcQuaid]

Thanks again @gromgit! Powershell can be a follow-up.

[jeremy]

When MANPATH is not set, this now triggers an unbound variable error in Bash shells with set -eu:

bash -eu -c 'unset MANPATH; eval "$(/opt/homebrew/bin/brew shellenv)"'
bash: line 5: MANPATH: unbound variable

Could use e.g.

echo "export MANPATH=\${MANPATH+:}:${MANPATH#:}${HOMEBREW_PREFIX}/share/man;"