Adjust X11 display search range to support larger x11_display_offset values

[Romk-a]

Description:
This pull request modifies the X11 display search logic in x11_create_display_inet to allow the x11_display_offset parameter to exceed 1000, addressing a limitation in the current implementation.
Problem
Currently, setting x11_display_offset to a value greater than 1000 prevents the function from entering the for loop, as the condition display_number < MAX_DISPLAYS (where MAX_DISPLAYS is 1000) fails immediately. Execution then proceeds to the check if (display_number >= MAX_DISPLAYS), resulting in an error: "Failed to allocate internet-domain X11 display socket." This behavior is incorrect because it unnecessarily restricts the usable range of x11_display_offset, limiting the port range to 6000–7000.
Solution
The changes in this PR adjust the loop condition from display_number < MAX_DISPLAYS to display_number < x11_display_offset + MAX_DISPLAYS, and update the subsequent check accordingly. This ensures the loop iterates over a range of MAX_DISPLAYS (1000) displays starting from x11_display_offset, rather than being capped at a fixed maximum of 1000. As a result, users can specify a higher x11_display_offset to select a more convenient port range (e.g., beyond 7000).
Safety
The modification is safe because the existing check x11_display_offset > UINT16_MAX - X11_BASE_PORT - MAX_DISPLAYS already enforces an upper bound. For X11_BASE_PORT = 6000 and MAX_DISPLAYS = 1000, this limits x11_display_offset to 58535, ensuring the maximum port (6000 + 58535 + 999 = 65534) stays within the valid range of 65535. Thus, these changes align with the existing safety constraints while improving flexibility.
Benefits
This adjustment allows users to configure x11_display_offset more freely, enabling X11 forwarding on a broader and more customizable range of ports, rather than being restricted to 6000–7000.
Let me know if further clarification or adjustments are needed!

[djmdjm]

Why do you want this? X11 ports usually are in this range

[Romk-a]

@djmdjm, I have a large server with virtualization deployed, and sometimes it happens that the ports overlap, which is why I have problems running the hypervisor and virtual machines. And I want to shift the ssh connection with x11 forwarding to about the 12000 port range.
Plus, for those who use the default configuration, the changes will not affect in any way.