Avoid conflicts in portstree when updating (or deleting)

  * Detects if a portstree is already used by poudriere, and refuses to
    update or delete this portstree.
More readable english.
Update default configuration file.
Rework a bit documentation.

  * remove duplicate informations, README now refers to the conf file
    instead of having a soon outated content

  * make the default configuration file more verbose
export SHELL
don't mix echo and cat
reorder check
define POUDRIEREPREFIX
simplify case
style
Add a new flag -o to tesport:  specify an origin.

With -o, we know that the port is inside the ports tree and dont mount it via
nullfs.
Also better use HOST_PORTDIRECTORY and PORTDIRECTORY.
TODO: error if both -d and -o are set
Rollback the behavior of poudriere ports.

   Shows usage when invocated without args.
Clarify a bit help for poudriere ports.
don't mix echo and cat
style
use uname -m only once
Sort-out extra files.
Do not list files in /wrkdirs
Sort files and dirs.

Do not list modified files as leftovers.
Ignore these damn nls folders!
Only list port trees root filesystems.

If a port tree contains a ZFS filesystem, it should not be listed in the
ports list
Log errors on... stderr (Yay!)

At least, we can see "No such ports tree foo" [1] when we are passing
the wrong ports tree name instead of just a message telling some signal
was caught [2] and some unrelated message [3]:

====>> Mounting devfs
====>> Mounting /proc
====>> Mounting linuxfs
====>> Adding IP alias
====>> Starting jail 8_2_RELEASE_amd64
No such ports tree p_romain [1]
====>> Signal caught, cleaning up and exiting [2]
/usr/local/share/poudriere/stop_jail.sh: ERROR: No such jail 8_2_RELEASE_amd64 [3]

Basically, any function which output is captured can now use err to emit
diagnostic messages.
Make 'a' switch behave correctly.

  * -a i386 on i386 arch used to fail
Also ignore deletes in wrkdirs
Use env to pass variables.
Adjust doc.
s/PORTSDIR/POUDRIERE_PORTSDIR/ to avoid collision