GUACAMOLE-2333: Allow the RDP redirected-drive open-file limit to be raised without a rebuild

[benjefferies]

Summary

Each RDP session currently allows at most 128 concurrent opens on the redirected drive. Directory listings, thumbnailers, and copies can fill that table. Further opens then fail with ENFILE (File open refused (-1)), and the drive appears unusable until the session is reconnected.

This change honours GUAC_RDP_FS_MAX_FILES when allocating the per-session table so the limit can be raised without rebuilding guacd. The compiled default remains 128.

What changed

• Parse GUAC_RDP_FS_MAX_FILES as a positive integer up to 4096
• Allocate the open-file table on the heap so its size is no longer a compile-time array
• Ignore malformed values and keep the default of 128
• Abort filesystem setup if the table cannot be allocated, instead of crashing on the next open
• Document the environment variable on the guacd Docker image README

Test plan

• Parser tests in src/protocols/rdp/tests/fs/max_files.c
• Unset env still refuses new opens around 128
• GUAC_RDP_FS_MAX_FILES=1024 allows more concurrent opens
• GUAC_RDP_FS_MAX_FILES=nope logs a warning and keeps 128

Fixes: https://issues.apache.org/jira/browse/GUACAMOLE-2333