zprofile: Expand the default list of well known paths
zprofile: Expand the default list of well known paths

Two additional sets of paths are now added to the default list of well
known paths: '$HOME/{bin,sbin}' and '/opt/{homebrew,local}/{bin,sbin}'.

- '$HOME/{bin,sbin}': Most users have custom scripts in '$HOME/bin'
anyway, we might as well honor those. '$HOME/sbin' is not really common,
but we can keep it for consistency.
- '/opt/{homebrew,local}/{bin,sbin}': With Homebrew changing default
installation location in macOS on Apple Silicon which will eventually
become ubiquitous, we have a good reason to add these paths by default.
While at it, we also honor MacPorts installation.

In all cases, we add them _iff_ the paths actually exist, not otherwise.
This has the side effect of a newly installed program not available
immediately in the '$path' in a mint fresh system (because of the fact
that '/opt/{homebrew,local}/{bin,sbin}' won't exist initially) until the
shell is reloaded. But that's a minor inconvenience to keep the '$path'
from getting unnecessarily bloated.

jamesacarr/prezto,hghwng/prezto,ParthKolekar/prezto,nstCactus/prezto,adamwalz/prezto,yyadavalli/prezto,vogler/prezto,jeffcox/prezto,wzxu/prezto,randomizedthinking/prezto,louis-hsu/prezto,indrajitr/prezto,tcbbd/prezto,sorin-ionescu/prezto,pedroxs/prezto