[stunnel-users] STRLEN in common.h may be too small

In common.h of stunnel-4.x, the parameter STRLEN (used for many buffers) is set to 256. This is insufficient in some cases, e.g., the CAPABILITY response of an IMAP server or the pathname of a file may be too long, resulting in errors. If you encounter such cases (e.g., messages "Input line too long"), increase STRLEN, e.g., to 512.