mail: Add checkmail routine

Added script to call for checking mail. Will run individual pre- and
post-hooks. (By default, imapfilter and notmuch update.)

Simple options can be changed through environment variables:

Hooks can be set to different scripts with MBSYNC_PRE and
MBSYNC_POST, respectively.

Will also run mbsync repeatedly, until it exits without error. By
default it will try running it 3 times before giving up, this can be set
with MBSYNC_MAX_TRIES.

No functional imapfilters have been added yet.

mbsync and imapfilter both adhere to the XDG base directory layout
(somewhat), checking for their option files in .config/{isync,imapfilter}
respectively.

Everything still pending changes, the mail eco-system is hard to
wrap one's head around.

mail/.config/isync/mbsyncrc

@@ -10,27 +10,26 @@ PassCmd "pass show misc/aerc-gmail-app-password | head -n1"
 #
 # Use SSL
 SSLType IMAPS
-# The following line should work. If get certificate errors, uncomment the two following lines and read the "Troubleshooting" section.
 CertificateFile /etc/ssl/certs/ca-certificates.crt
-#CertificateFile ~/.cert/imap.gmail.com.pem
-#CertificateFile ~/.cert/Equifax_Secure_CA.pem
-PipelineDepth 50
+# Throttle simultaneous access to make google happy
+PipelineDepth 60
 
 IMAPStore gmail-remote
 Account gmail
 
-MaildirStore gmail-local
+MaildirStore mail-local
 Subfolders Verbatim
 # The trailing "/" is important
 Path ~/documents/mail/
 Inbox ~/documents/mail/Inbox
+# define generous maximum size to store locally
+MaxSize 50M
 
-Channel gmail
+Channel gmail-general
 Far :gmail-remote:
-Near :gmail-local:
+Near :mail-local:
 # Exclude everything under the internal [Gmail] folder, except the interesting folders
-Patterns * ![Gmail]* "[Gmail]/Sent Mail" "[Gmail]/Starred" "[Gmail]/All Mail"
-# Or include everything
+Patterns * ![Gmail]* "[Gmail]/Sent Mail" "[Gmail]/Starred" "[Gmail]/All Mail" "[Gmail]/Trash"
 #Patterns *
 # Automatically create missing mailboxes, both locally and on the server
 Create Both